1. Field of the Disclosure
The present disclosure relates to reduced large particle counts in slurries used in chemical mechanical polishing. In particular, the present disclosure relates to the purity of the copper removal rate enhancer used in the slurry, typically an amino acid or derivative thereof.
2. Description of the Related Art
The process known as chemical mechanical polishing or planarization (CMP) involves the planarization of different layers on semiconductor wafers using a polishing pad and slurry to polish away excess or unwanted layers of materials prior to construction of subsequent layers. Due to its extraordinary conductivity, copper is a commonly used material for forming interconnects in semiconductor manufacturing. Once a copper inlaid structure is formed by, for example, a damascene process which deposits copper according to the pattern dictated by a stenciled template on the substrate, isolated copper wires are made by polishing and clearing copper and diffusion barrier metal between the inlaid wires. Copper and barrier layer CMP involves the repeated cycle of copper and barrier layer polishing for each layer as the back end of line layers of the chip are fabricated. It is desired to polish the wafers at a high removal rate of material to enhance throughput, while still maintaining favorable wafer characteristics such as a low number of overall defects, especially scratches which are thought to be caused in part by undesirable foreign particles in the slurry. The presence of these undesirable particles is typically monitored by light scattering techniques to determine large particle counts (LPCs), in which the concentrations of particles with diameters above a selected threshold, for example 0.5 micrometers (μm) or 1.0 μm, are quantified in solution. The selected size threshold is typically well above the 99th percentile for the size distribution of desired particles in solution and thus prohibits confusion as to whether or not particles contributing to LPCs in slurries are undesirable or desirable.
A typical copper CMP process utilizing copper slurry consists of 2 process steps which have been described in detail in the prior art such as in U.S. Pat. No. 6,083,840 (Mravic, Pasqualoni, Mahulikar). First, the electroplated copper overburden is polished down at a relatively high down force in a bulk polish step which removes the majority of the overburden. Subsequently, that remaining copper overburden from the first step is polished off at a lower down force, with a stop on the barrier layer. This step can be combined with the first step, depending on the polisher type or configuration. The goal is to clear all copper from the barrier material while avoiding a variety of defects. Namely, undesirable deep scratches from the copper CMP steps may persist during later stages of chip fabrication if they are of sufficient depth not to be removed in subsequent barrier polishing. These types of scratches can ultimately compromise device performance, and one of the potential culprits behind deep scratches would be large undesirable particles present in the copper slurry. As a result, minimizing LPCs in copper slurries continues to be a goal, particularly as device features continue to shrink at more advanced nodes and even the smallest of scratches can be detrimental to device performance and thus die yields on wafers. LPC minimization is especially important as slurries are made at higher and higher concentrations prior to being diluted by the end user by a rather large factor such as 20×, as described in U.S. Pat. No. 8,192,644 (Kim, Wen).
LPCs in CMP slurries primarily come from the abrasives used in the slurries. Typical abrasives are colloidal silica, fumed silica, ceria and alumina. In practice a process of filtration is used to remove particles larger than certain size. The prior art involves utilization of size exclusion filtration to remove physical impurities from copper slurries which contribute to LPCs. U.S. Pat. No. 6,749,488 (Mahulikar, Lafollette) describes the relationship of LPCs to defects and gives a way to control LPCs by filtration. While the contribution of abrasives to LPCs is well established, the contribution of other slurry chemicals to the LPCs is not well known. These aqueous chemicals are fully dissolved in the slurry formulation and hence may not have a significant effect on LPCs. But there could be complex factors in play. Certain chemicals can alter surface charges on the particles causing them to agglomerate, creating in situ LPCs even after filtration. Chemicals can also influence the wetting capability of filters changing the filtration efficiency. Finally they can affect the dissolution kinetics causing precipitation of species adding to LPCs. These secondary problems exist in cu slurry formulations. With the semiconductor industry demanding tighter control limits and very low variations these LPC issues must be addressed.
The use of high purity particles and high purity slurries is described in U.S. Pat. No. 8,211,193 (Mahulikar, Wang). The effect of purity on various functional performance related parameters is described, including defectivity. The functional role of the removal rate enhancer (RRE) is described in detail in the prior art such as in U.S. Pat. No. RE 37,786 (Hirabayashi, Higuchi), but the prior art fails to report purity effect of is specific chemicals in the slurry formulation on the LPCs or defects. Thus, one objective of this disclosure is to address the problems of chemical effects on the LPCs in a copper slurry in general and the effect of the purity level of RREs on the LPCs in particular. There is also a need to determine if conductivity can be a reliable indicator of how impure a slurry is, which could relate to how many defect-causing impurities are present in the slurry and if a particular slurry is likely to be prone to cause defects during copper CMP, especially deep scratches. In addition, if conductivity contributed to solution by the RRE is found to correlate with LPCs and thus how many potentially defect-causing particles are present in solution, restrictions on setting minimum purity levels of the RRE should be made.